1. Field of the Invention
The present invention relates to a steel cord for the reinforcement of rubber articles comprising a core consisting of two to four steel filaments, and one layer of steel filaments around the core. The layer consists of filaments which face two filaments of the core and which form an inner-sheath, and of filaments which face only one filament of said core and which form an outer-sheath. All these filaments are twisted in the same direction and at the same pitch.
2. Description of the Related Art
A steel cord for the reinforcement of rubber articles conveniently comprises steel filaments having a carbon content of more than 0.60 per cent by weight (e.g. more than 0.65% 0.78%, 0.82% or 0.90%). A typical steel composition is:a minimum carbon content above 0.65% a manganese content between 0.40% and 0.70%, a silicon content between 0.15% and 0.30% and a maximum sulphur and maximum phosphorus content of 0.03%, all percentages being percentages by weight. Other elements such as chromium or boron or vanadium may also be alloyed.
The diameter of such steel filaments lies in the range of 0.05 mm to 0.80 mm, preferably in the range of 0.15 mm to 0.40 mm (e.g. 0.23 mm, 0.26 mm or 0.32 mm).
The steel filaments are usually provided with a coating which promotes the adherence of steel wire to rubber articles.
Such a coating conveniently comprises copper, zinc, brass or ternary brass alloy, or a combination of two or more different layers thereof. The thickness of the coating ranges from 0.05 to 0.40 micron, preferably from 0.12 to 0.22 micron. The coating may also be present in the form of a thin film of chemical primer material for ensuring good rubber penetration and adhesion.
A steel cord with all the filaments - except for the wrapping filament--twisted in the same direction with the same twist pitch is disclosed e.g. in GB-A-2 028 393 and is know as a compact cord.
Advantages of such a steel cord are its economical way of manufacturing (in one step), its compact form which allows much steel per cross-sectional surface unit and its line contacts.
Such a steel cord, however, suffers from fretting wear between the filaments of the layer and from core migration, i.e. the filaments of the core slips out of the cord due to repeated bends.
The prior art has already provided several solutions for avoiding core migration.
First of all, core migration may be avoided by differing the twist pitch of the filaments of the core substantially from the twist pitch of filaments of the layer and by increasing the diameter of the core filaments with respect to the diameter of the layer (U.S.-A-4,627,229).
In U.S.-A-4,783,955 another solution is proposed. The diameter of the core filaments is increased with respect to the diameter of the filaments of the layer while the twist pitch of the core filaments remains the same as the twist of the filaments of the layer.
The latter solution is based on the reasoning that in order to avoid core migration two measures must be taken:
1. the filaments of the layer must apply a tightening force to the core filaments, and PA1 2. rubber must sufficiently penetrate into the inside of the cord.